316 MORPHOLOGY AND EVOLUTION or they undergo progressive degeneration as the result of loss of function. The external ear of the Apes and Man is a good example of an organ which has undergone changes of the latter type; and Wallis (412a) has aptly likened it to an outpost, once important, but no longer essential, from which the garrison is withdrawing. Keith (100) points out that these retrogressive changes have proceeded farthest in the Orang; they are not so marked in Man and the Gorilla; and they are least evident in the Chimpanzee. On the other hand, Wallis states that the human ear is most ancestral in type. The size of the ear is related to habits. The Gibbons and Chimpanzees have the largest ears, for they require quick hearing to enable them to escape from enemies when they are on the ground. The Orang has the most degenerate ear for it leads a safe life among the branches of high trees. The Gorilla is less arboreal in its habits than the other Anthropoids, but its ears are small. It is, however, a most formidable animal and can defend itself when attacked, so acuteness of hearing is of no great consequence. Pocock (223) states that " it is important to note that of the two Apes inhabiting W. Africa, namely the Chimpanzee and Gorilla, and of the two inhabiting the East Indies, namely, the Orang and the Gibbon, the larger and stronger has in each case small, insignificant ears, and the smaller and weaker large ears." The external ears are most prominent in Man and the Chimpanzee, for they are concealed by the hairs in the Gibbons, Orang and Gorilla. Their small size in the Gorilla and Orang permits them to be easily hidden.